The present invention relates to a method for measuring the concentration of nitrogen in argon by means of ionization mobility spectroscopy.
Argon is widely used in the semiconductor industry, both as a transport gas wherein reactive species are diluted, and as a support gas for plasma formation in cathodic deposition processes (better known in the field with the definitions of xe2x80x9csputteringxe2x80x9d or xe2x80x9cPhysical Vapor Depositionxe2x80x9d, PVD). The purity of the employed argon is very important; as a matter of fact contaminants possibly present in the reagents or in the reaction environment can be incorporated into the solid state devices, altering the electrical or magnetic properties thereof and thus giving rise to production wastes.
Argon purification is the subject-matter of a number of patents, including for example patent GB-B-2,177,079 in the applicant""s name. According to this patent, argon is purified by passing it through a bed of a getter material (an alloy based on zirconium, vanadium and iron) kept at a temperature between 350 and 450xc2x0 C. By this method the content of impurities in the argon is reduced below 1 part per billion (ppb, equivalent to one molecule of impurities per 109 argon atoms).
In these conditions, it is also necessary to have the possibility of checking the gas purity and its consistency in time, so as to detect increases of impurity concentration, due for example to anomalies in the operation of the purifiers, loss of tightness of the gas lines or the like.
A particularly interesting technique for carrying out this analysis is ionization mobility spectroscopy, better known in the field with the abbreviation IMS (the same abbreviation is used also for the instrument with which the technique is carried out, in this case indicating xe2x80x9cIonization Mobility Spectrometerxe2x80x9d). The interest in this technique derives from its high sensitivity, associated with the limited size and cost of the instrument; by operating in appropriate conditions it is possible to sense gas or vapor phase species in a gaseous medium in quantities of the range of picograms (pg, that is 10xe2x88x9212 grams), or in concentrations of the order of parts per trillion (ppt, equivalent to a molecule of analyzed substance per 1012 gas molecules of the sample). IMS instruments and the methods of analysis employing these are described, e.g., in U.S. Pat. Nos. 5,457,316 and 5,955,886 in the name of U.S. company PCP Inc.
An IMS instrument is essentially formed of a reaction zone, a separation zone and a detector of charged particles.
In the reaction zone takes place the ionization, commonly by means of beta-radiations emitted by 63Ni, of the sample comprising the gases or vapors to be analyzed in a transport gas. Due to the ratio between the number of molecules of the main gas and the impurities therein, the first ionization acts essentially take place on the former, with the formation of the so-called xe2x80x9creagent ionsxe2x80x9d: the charge of these ions is then distributed to the other present species as a function of their electronic or proton affinities or of their ionization potentials. For an illustration of the (rather complex) charge transfer principles which are the base of the ionization mobility spectrometry technique, reference can be made to the book xe2x80x9cIon Mobility Spectrometryxe2x80x9d by G. A. Eicen an and Z. Karpas, published in 1994 by CRC Press.
The reaction zone is divided from the separation zone by a grid which, kept at a suitable potential, prevents the ions produced in the former zone from entering into the latter zone. The moment when the grid potential is annulled, thus allowing the entrance of the ions in the separation zone, is the xe2x80x9ctime zeroxe2x80x9d of the analysis. The separation zone comprises a series of electrodes which create an electric field such that the ions are carried from the reaction zone towards the detector. This zone is kept at atmospheric pressure. Therefore, the velocity of motion of the ions depends on the electric field and on the crosssection thereof in the gaseous medium. By registering the reading of current of the particle detector as a function of the time passed from the xe2x80x9ctime zero,xe2x80x9d peaks corresponding to the so-called xe2x80x9ctime of flightxe2x80x9d of the different ions are obtained. From the determination of the time of flight, it is possible to go back to the presence of the substance, object of the analysis.
In spite of its conceptual simplicity, the application of the technique involves some difficulties in the interpretation of the analysis results.
The instrument, similarly to chromatographs, provides as result of the analysis the crossing time (time of flight in the case of the IMS) of the present species, but does not provide further indications of the chemical nature of the species corresponding to each peak.
For attributing each peak to a chemical species, the IMS may be connected to a mass spectrometer, which determines the chemical nature of each ion, but in this way the above mentioned advantages of low cost and compactness are lost.
Alternatively, it is possible to run calibration tests on samples formed of an extremely pure transport gas containing a substance to be subsequently analyzed, thus determining the time of flight of the molecules of interest. The analysis under real conditions is, however, complicated due to the simultaneous presence of more substances, giving rise to various ionic species which nay lead to phenomena of charge transfer among each other or with present neutral molecules, so that the times of flight found in the analysis can be characteristic of species different from those whose presence is to be determined. In the particular case of the analysis of traces of nitrogen in argon, this is essentially impossible to carry out directly, because the charge transfer among the Ar+ ions (the first ionization product) and nitrogen is scarcely efficient.
In order to overcome the problems found in the real analyses, it has been developed the method of adding the sample gas with a specific substance, called xe2x80x9cdoping gasxe2x80x9d which, according to various mechanisms, obtains the effect of notably increasing the sensitivity of the measure towards the specific molecule object of the analysis.
As examples of practical application of the method of the doping gas may be mentioned U.S. Pat. No. 4,551,624, regarding the addition of ketones or halogenated gases to the gas to be analyzed; U.S. Pat. Nos. 5,032,721 and 5,095,206 regarding, respectively, the use of phenols and sulphur dioxide in the analysis of acid gases; and U.S. Pat. No. 5,238,199, regarding the use of amines in the analysis of chlorine dioxide.
In the specific literature of the IMS field, however, there are no examples of the use of a doping gas for measuring nitrogen in argon. From the paper xe2x80x9cDetection of trace nitrogen in bulk argon using proton transfer reactionsxe2x80x9d, by E. J. Hunter et al., (Journal of Vacuum Science and Technology, section A, vol. 16, No. 5 of September-October 1998, pages 3127-3130) it is known that the addition of hydrogen in concentrations up to 2xe2x80x943% increases the sensitivity of an analysis of nitrogen in argon carried out with the technique of mass spectrometry with chemical ionization at atmospheric pressure (technique known, like the relevant instrument, with the abbreviation APCI-MS). This technique provides results which are intrinsically simpler to interpret than the IMS technique, because the detector is a mass spectrometer, which distinguishes the ions present in the sample on the base of the mass/charge ratio thereof, and therefore directly attributes the chemical nature to each measured signal. Besides, the content of this article includes a method for the calibration of the APCI-MS instrument with pure gases, at most containing traces of gases which have to be subsequently analyzed. The teachings of this paper however, if applied straightforwardly, do not allow the IMS measure of nitrogen in argon; as the inventors have observed. By using the very same system described in the article above in an IMS analysis, the measure of nitrogen is quantitatively unreliable, and, in the worst cases, the analysis output is a null reading of nitrogen also in the presence of this gas in the sample, so that even the simple qualitative analysis is impossible.
An object of the present invention is to provide a method for measuring the concentration of nitrogen in argon by means of ionization mobility spectroscopy.
This object is achieved according to the present invention by a method comprising the steps of:
providing an argon flow sample to be analyzed comprising a total concentration of impurities other than nitrogen not higher than about 1 ppb;
adding a quantity of hydrogen to the argon flow sample to be analyzed to produce a mixture of gases comprising a hydrogen concentration between 0.005 and 100 parts per million by volume; and
analyzing the mixture of gases with an ion mobility spectroscopy analyzer.
Analogously to what was reported in the cited article by E. J. Hunter et al., the reaction of charge transfer at the base of the measure is believed to be the following:
ArH++N2xe2x86x92N2H++Arxe2x80x83xe2x80x83(I)
The studies of the reactions which take place in an IMS chamber have demonstrated that the species really present are more complex than the above reported ones, and generally consist in complex aggregates of ions and neutral molecules. For example, in the method according to the present invention, species of the type (Ar)nH+, (Ar)p(N2)qH+ and (H2O)r(Ar)sH+ are involved, where n, p, q, r and s are integer numbers. However, xe2x80x9csimplifiedxe2x80x9d formulas will be used in the following for the sake of simplicity; some simplified formulas corresponding to the above mentioned species and used in the following are, respectively, ArH+, N2H+, (H2O)2H+ and H3O+.